This invention relates to instruments for measuring the conductivity of thin layers or coatings of electrically conductive material within apertures or holes and particularly to instruments for measuring the plating thickness or conductivity of electrically conductive material on the walls defining a through hole in printed circuit boards.
Two pairs of electrodes, one pair on each side of the through hole are used for measuring the conductivity of plated through holes. One electrode of each pair is used for injecting current into the through hole and the other electrode of each pair is used for measuring the resulting voltage drop across the through hole. This technique is old in the art and is commonly referred to as the Kelvin technique or four-point measurement technique.
Electrical probe assemblies of various configurations are known such as described in U.S. Pat. No. 902,753; U.S. Pat. No. 2,142,619; U.S. Pat. No. 2,208,023; U.S. Pat. No. 2,437,697; U.S. Pat. No. 3,042,862; U.S. Pat. No. 3,192,474; U.S. Pat. No. 3,335,358; U.S. Pat. No. 3,676,776; British Pat. No. 922,225; W. Gee et al. A Versatile Over-Under Four-Point Probe Apparatus, J. Physics E. (GB), January, 1971, pp. 70-72; and R. M. Murcko, Split-Tip for Terminal Resistance Measurements, IBM Technical Disclosure Bulletin, July, 1971, p. 479.
Instruments for measuring the effective thickness of through hole plating in circuit board workpieces have been developed such as U.S. Pat. No. 3,776,470; U.S. Pat. No. 3,885,215; U.S. Pat. No. 4,042,880; and British Pat. No. 1,244,572. Apparatus of the type described in these patents utilize four-point techniques to measure the conductivity of through holes. These types of electrode assemblies are characterized by having an electrode tip divided into a current injection portion and a voltage measurement portion which are fixed relative to one another.
Electrical probe assemblies for measuring resistance having spring biased electrodes are described, for example; in U.S. Pat. No. 2,885,648 and U.S. Pat. No. 3,229,200. However, with the two types of electrodes shown, it is not possible to get substantially a full 360.degree. current injection by the current electrode and also at the same time insure good contact for the voltage electrode with the through hole.